Pretty much all I shoot is multiple exposures, take a look at my gallery.
If you try to use a lot of rules and formulas game plans etc. you will miss out on the best part of what multiple exposed imagery can give you, its magical-ness. The random uncontrolled, un-conceived mixing of imagery/light is what gives the multiple exposure its uniqueness. I suggest the "go by feel method" using the standard practices you normally would use remembering with each pop the exposure values are adding up. Be open to experimenting and you will hit on something that will fall into place and surprise you. I tell my friends that if you want the best of what Multiple Exposure can be don't calculate your every move with exact precision. If "Precision/Exactness/Documentation" is what your looking for shoot Landscapes... 😀